This invention relates to a solar energy absorber and more particularly to a solar energy absorber which traps the visible spectrum of electromagnetic radiation for heating a fluid.
Heretofore, most systems designed to trap solar energy utilized a system often called "the green house effect." In such systems, visible radiation passes through one or more transparent outer covers and is almost entirely absorbed by an absorptive surface. The absorptive surface was usually constructed of tubing wherein a fluid must pass therethrough for transferring thermal energy absorbed by conduction. The outer cover, while transparent to visible radiation is largely opaque to infrared radiation off of the absorber surface, and hence, absorbs it. It conducts the energy that is not re-radiated to the absorber surface through the outer cover where it is convected or re-radiated away. Examples of such a system are shown in U.S. Pat. No. 1,258,405 and U.S. Pat. No. 2,969,788.
For a given absorptive surface, the amount of energy lost by convection and infrared radiation is a function of the outer cover temperature. Often more than one outer cover is used to reduce the infrared energy reaching and, consequently, heating the final cover.
The use of transparent covers, as described, is not without its penalties however, in a practical absorber system design. The transparent covers reflect some incoming radiation away before it gets to the principal absorptive surfaces. It also absorbs some of the incoming visible radiation. The transparent covers are usually of a brittle material, glass or quartz. Such are consequently hard to handle in assembly and maintenance. Dust adhering to such a cover increases losses.
Another problem with the transparent covers is that if the temperatures rise above a predetermined level, such will often be damaged by breaking or the like.